“Some other stakeholders GAO interviewed identified potential reliability challenges,” the report said. “Among other things, it may be difficult to schedule and complete all retrofits to install controls and to resolve all potential reliability concerns associated with retirements within compliance deadlines.”

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The rules GAO studied were: the Cross-State Air Pollution Rule; the Mercury and Air Toxics Standards; the proposed Cooling Water Intake Structures regulation; and the proposed Disposal of Coal Combustion Residuals regulation.

Republicans have denounced the EPA policies. They say procuring the technology needed to comply with the rules will take longer than EPA is willing to give.

GOP lawmakers, and the electric power industry, contend the rules will shutter too many coal-fired plants, potentially leading to more electricity blackouts. They also say it will increase electricity bills for residents in coal-heavy states.

The GAO report partially refuted the cost claim. It said the rules would increase rates unevenly — the coal-reliant South being hardest hit — but that over time costs could decrease compared with historical fluctuations.

The rise of natural gas will also eliminate some of the cost and reliability concerns, the GAO report said. Cheap natural gas is already replacing coal as a source of electric power, and that alone could push power operators to shut down coal plants, the report said.

Republican staff for the House Energy and Commerce Committee, which has held several hearings on the rules and their effects on electric reliability, told The Hill it was still reviewing the report.

The GAO report builds on an August 2011 report by the nonpartisan Congressional Research Service (CRS) that said utility industry claims about the rules were excessive. That report found the regulations would not result in a significant number of power plant retirements.

The CRS report also said public health benefits from instituting the rules far outweighed the costs.